The present invention relates to catalytic converters useful for the treatment of combustion exhaust emissions from internal combustion engines. More particularly, the invention relates to improved honeycomb catalyst supports, and a support system for mounting them, which provides a mounted converter system with excellent resistance to high temperature thermal damage, and to shock and vibration damage, at reduced cost and with reduced mount complexity.
A continuing problem encountered in the development of mounting systems for catalytic converters relates to the temperature stability of the mounting. This problem is particularly severe in so-called "close-coupled" catalytic converters, sometimes referred to as pre-converters, which are typically located very near to the engine in the engine exhaust system. This proximity to the engine exposes the catalyst, ceramic honeycomb substrate, and converter mounting system to significantly higher exhaust temperatures and vibrational loads than are encountered by converters in more conventional automotive underbody locations.
Customarily, intumescent mats have been used as essential mounting materials for supporting ceramic substrate catalytic converter in metal enclosures or "cans". These mats are composed of mineral constituents selected so that, as mat temperatures increase during first use, the mat expands and thereby secures the substrate in the converter can. The use of intumescent mat mounting materials alone and with other fiber mat materials to restrain ceramic honeycombs within metal converter enclosures is described, for example, in U.S. Pat. Nos. 4,863,700 to Ten Eyck and 5,376,341 to Gulati.
Disadvantageously, conventional intumescent mat materials tend to degrade at temperatures above about 700.degree. C. With this degradation, the retention pressure exerted by the mat on the substrate decreases and the potential for axial substrate movement under exhaust backpressure, and eventual failure of the mounting system, increases. In the severe thermal and vibrational environment of the automotive exhaust pre-converter, thermal degradation of the mat becomes an increasingly serious concern.
Discussions of these mounting problems are presented in SAE Paper No. 952414 and SAE Paper No. 960563. In general, the experiments detailed in these papers indicate that the residual shear strength of conventional mat mount systems decreases dramatically at environment temperatures in the 950.degree. C. to 1050.degree. C. range. Between 1000.degree. C. and 1050.degree. C. the residual shear strength can fall below accepted minimum strength levels, levels which should provide a safety factor 3 to 4 times the calculated pressure (15 kPa) required to cause axial substrate displacement in a hot (950.degree. C.) high-acceleration (75g) vibration environment. In addition to thermal problems, the mechanical degradation of mat mounting materials under these vibration conditions can also be expected.